I work at a relatively busy international airport that has military, airline, and GA traffic. I see the "plane peepers" everyday. That being said I have also witnessed suspicious behavior from some of the peepers that would probably be worth a closer look.
An example of such behavior: While working very late on a server that had crashed I worked throughout the night. I would go outside the hangar to take a smoke break every couple hours. When I went out the front door a car I had never seen before suddenly backed up and drove off in a hurry. The driver looked frantic. I continued on just noting that it was odd.
This same car did the same thing multiple times that night, as soon as I appeared at the front door, the camera was thrown in the passenger seat and he peeled out.
The general gist of the poster is report behavior that may be suspicious (although that is subjective). How else can you convey that in a picture with a telephoto lense and a hood. ;)